This invention relates to a steered adaptive antenna arrangement for enhanced reception of constant envelope signals.
Recent work has shown how the misalignment sensitivity problem associated with steered adaptive arrays can be reduced by appling a limit on the computed weight update. A possible scheme is shown by FIG. 1. Here, the summed output is correlated with each element signal, applied to the limiter and added to the steering component. The derived value is then used to drive the associated weight coefficient. As indicated by the diagram, the limiter preserves phase information and simply restricts the modulus of the weight update component. Other forms of limiter can however be devised.
FIG. 2 illustrates the scheme simplistically in terms of the steering vector beam pattern and a "retro-beam" (derivable from the weight update vector) formed by the adaptive process. In principle, the system cancels the received signal by adjusting the direction and level of the retro-beam to match the response from the steering vector beam. By applying a modulus limit on the retro-beam gain, we can effectively prevent the array from cancelling any signal arriving from an angular sector close to peak of beam. For example, in the simulation results presented later on, a weight update limit of 0.7 times the modulus of the corresponding steering vector component gave rise to a protected zone of approximately one half of a beamwidth.
Whereas this technique can be shown to perform well under many circumstances, it does however suffer two significant problems caused by the presence of the desired signal in the adaptive process. These are:
(i) the method necessitates the use of low update gain factors (and hence implies relatively slow convergence) to maintain low weight jitter and an acceptable signal to noise ratio. PA1 (ii) the desired signal can "capture" the limiters and lose adaptive degrees of freedom causing degraded nulling in the presence of multiple jammers. PA1 single jammer (Gaussian envelope, OdBe at 45.degree. rel. boresight. PA1 wanted signal (constant envelope), -10 dBe at 0.degree., 5.degree., 9.degree., 9.5.degree. and 1O.degree. for FIGS. 3(a) to 3(e) respectively. PA1 6 element linear array, d/.lambda..perspectiveto.0.5. PA1 boresight steering vector. PA1 thermal noise floor, -50 dBe. PA1 update gain factor, 0.1.
To illustrate the first aspect, it can be shown that the fractional increase in error residual power .beta., due to random weight jitter ignoring the effect of the weight update limiter is EQU .beta..varies.GNP.sub.tot
where N is the number of elements, G is the update gain factor and P.sub.tot is the total power at each element of the array. Since the mean residue at steady-state will be dominated by the desired signal, then the inverse of the .beta. factor indicates in effect the resultant signal to noise ratio at the beamformed output. Hence, maintaining low weight jitter becomes much more critical when adapting in the presence of the wanted signal. For example, if a 20 dB resultant signal-to-noise ratio (SNR) is required then the update gain factor must be set at a value some hundred times below the stability threshold (c.f. adaptation in the absence of the desired signal where a stability margin of 10 gives an acceptable weight jitter performance for most practical situations). In practical terms this could relate to a tenfold reduction in convergence rate.
FIGS. 3(a) to (e) illustrate the convergence of the steered processor for the following parameters;
The results show the progressive cancellation of the desired signal as it becomes increasingly misaligned from the steering direction. Weight jitter performance (reflected by the achieved signal to jammer plus noise ratio) is slightly better than than predicted by the earlier equation. (This must be attributable to the limiting operation).